Validation of the 18 kiloDalton Translocator Protein (TSPO) as a Novel Neuroimmunodulatory Target

Inclusion Criteria: 1. Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form. 2. Aged 35-65 years old 3. A female subject is eligible to participate if she is a) of non-childbearing potential, defined as pre-menopausal females with a documented tubal ligation or hysterectomy, or postmenopausal defined as 12 months of spontaneous amenorrhea or b) of childbearing potential but not pregnant (as determined by urinary pregnancy test on screening and on each study day) and willing to use one of the contraception methods listed below 4. Male subject must agree to use one of the contraception methods listed above. 5. Willing to abstain from alcohol for the duration of dosing. 6. Expanded Disability Status Scale (EDSS) \>3.5 \<6.5 (SPMS patients only) Exclusion Criteria: 1. History of active neurological disease other than migraine or MS 2. Clinically meaningful abnormalities in routine bloods including: * eGFR \< 60ml/min * Elevation of liver enzymes/bilirubin * Prolonged prothrombin time * Thrombocytopenia 3. Use of the following medications or therapies: * Immunosuppressive or immunomodulatory drugs within the last 6 months * Alemtuzumab or haematopeotic stem cell therapy * Central nervous system depressants (including opioid analgesics, barbiturates, sleeping pills, antihistamines, antipsychotics) * P450 CY3A4 inducers or inhibitors * oral contraceptives * oral anticoagulants or antiplatelet agents other than low dose aspirin * levothyroxine 4. Currently breastfeeding 5. Any clinical significant medical conditions that in the opinion of the investigator would compromise subjects' safety or compliance with study procedures. 6. History of any clinical condition which in the opinion of the principal investigator would compromise the scientific integrity of the study, such as some chronic systemic diseases affecting blood, liver or kidneys or endocrine system 7. Unwillingness or inability to follow the procedures outlined in the protocol 8. Subject is mentally or legally incapacitated 9. Contraindication to XBD173 use: • Hypersensitivity to the active substance or to any of the excipients 10. Contraindication to etifoxine use: * Myasthenia gravis * syndromes of glucose and galactose malabsorption or lactose deficiency